1. Field of the Invention
The present invention relates to a process of cooling a heated polymer film by conveying the film through a body of cooling liquid and next removing cooling liquid that remains adherent to the film after the film has left the cooling liquid body. The invention relates in particular to a process of longitudinally stretching a molecularly orientable polyethylene terephthalate film.
2. Description of the Prior Art
In the extrusion process of making polymer film, the molten polymer is extruded through a slot-like orifice and the extruded polymer is received on a quenching drum on which the temperature of the extruded film is lowered sufficiently rapidly through the softening range of the polymer to obtain a substantially amorphous film. By stretching the quenched film longitudinally and transversely while the film is at the lower end of the softening range, above the second order transition temperature of the polymer, the film can be subjected to molecular orientation leading to an improvement in various physical properties of the film, notably the tensile strength.
The longitudinal stretching of the film is usually achieved by passing the film first around a series of slowly rotating hold back rollers and then around a series of rollers which are rotated more rapidly, and by heating the film between the two series of rollers to a temperature such that the film undergoes plastic elongation under the traction forces imposed on it due to the different speeds of the two series of rollers.
The cooling of the film after stretching is effected by cooling the more rapidly rotating rollers to a temperature well below the second order transition temperature of the film. The described steps of heating cooling and stretching a film are disclosed in U.S. Pat. No. 4,093,695 relating to a process for making polymeric film.
This process shows certain disadvantages the most important of which are as follows.
The stretching span, that is the length of the film path over which the stretching occurs, is rather long and in consequence there is appreciable "necking-in" of the film which causes beaded edges of the film.
Further, the alternating contact of the film with the different cooling rollers causes curving stresses in the film which produce a non-uniform heat transfer across the width of the film.
Finally, there is a deterioration in the surface quality of the film owing to the contact of the film with traction rollers while the film is under stretching forces.
There has been provided an improved longitudinal stretching process which considerably reduces the disadvantages mentioned hereinbefore. This process comprises preheating the film for causing the film temperature to increase, but insufficiently high for plastic film elongation to occur, then stretch-heating the film to a temperature at which instant plastic elongation occurs under the stretching forces, and finally very rapidly cooling the film by immersing the film in a bath of cooling water. This process is disclosed in EU B1 0 022 278.
While this process enables to overcome satisfactorily the mentioned difficulties, it shows in practice the disadvantage that the production speed is limited because above a certain film speed, it becomes difficult to satisfactorily remove the cooling water from the film.
As a matter of fact, it has been shown that the removal of cooling water by passing the film over successive squeeze rollers does not lead to a complete removal of the water, and that the removal of the rest water that adheres to the film in the form of isolated droplets or islands of liquid, by means of air knives or the like, is very difficult, not if impossible. The removal of cooling water from the film by squeeze means in the form of rubber bars does not produce better results. The removal of the cooling water by means of air knives produces slightly better results but still leads to production speeds which are limited in practice to approximately 50 m.min-1. It is of course possible to remove remaining water from the film by drying, but this is not desirable in practice since this step requires a large amount of energy. Moreover, the drying of the film causes drying stains on the film surface which form a problem in case the film will be used as a photographic support that will bear a transparent image.